DISCLOSED ARE SOLID FORMS OF 1-[(2-CH1ORO-3-THIAZO1Y1)METHYL]3{3,5-DICHLOROPHENYL)E2EHYDROXY4-METHYL-4-OXO-4H-PYRIDO[1 ,2-A]PYRIMIDINIUM INNER SALT (COMPOUND I). METHODS FOR THE PREPARATION OF SOLID FORMS OF COMPOUND I AND FOR THE CONVERSION OF ONE SOLID FORM OF COMPOUND 1 INTO ANOTHER ARE DISCLOSED. DISCLOSED ARE COMPOSITIONS FOR CONTROLLING AN INVERTEBRATE PEST COMPRISING A BIOLOGICALLY EFFECTIVE AMOUNT OF A SOLID FORM OF COMPOUND 1 AND AT LEAST ONE ADDITIONAL COMPONENT SELECTED FROM THE GROUP CONSISTING OF SURFACTANTS, SOLID DILUENTS AND LIQUID CARRIERS. COMPOSITIONS COMPRISING A MIXTURE OF A SOLID FORM OF COMPOUND 1 AND AT LEAST ONE OTHER NEMATOCIDE, INSECTICIDE AND/OR FUNGICIDE ARE ALSO DISCLOSED. ALSO DISCLOSED ARE METHODS FOR CONTROLLING INVERTEBRATE PESTS COMPRISING APPLYING TO A PLANT OR SEED, OR TO THE ENVIRONMENT OF THE PLANT OR SEED, A BIOLOGICALLY EFFECTIVE AMOUNT OF A SOLID FORM OF COMPOUND 1. (THE MOST ILLUSTRATED DRAWING IS
